Access operator's property with python
Passing properties to an operator, here's a small example (see the test call towards the end). This snippet is an elaboration on TextEditor > Templates > Python > Operator Simple

import bpy


class SomeOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.some_operator"
    bl_label = "Simple Object Operator"

    property_one = bpy.props.StringProperty(default='state one')
    property_two = bpy.props.StringProperty(default='state two')

    @classmethod
    def poll(cls, context):
        return True

    def execute(self, context):
        print(self.property_one)
        print(self.property_two)
        return {'FINISHED'}


def register():
    bpy.utils.register_class(SomeOperator)


def unregister():
    bpy.utils.unregister_class(SomeOperator)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.object.some_operator(property_one='woop_one', property_two='woop_two')


if you want to call such an operator from a layout element..

# if you want to pass a single property
row.operator('object.some_operator').property_one = 'some_new_value'

# if you want to pass multiple properties
some_op = row.operator('object.some_operator')
some_op.property_one = 'some new value one'
some_op.property_two = 'some new value two'




Autocomplete

If you don't know the name of the properties, then you can use the Python console's autocomplete.

In this example hit ctrl+space after the parenthesis ( , here you see lines is the only property you can set.



Documentation

Another resource is the current api docs use the search feature, type in for example bpy.ops.text.scroll and it will show the signature.

In the case of WM_OT_append you should be looking for bpy.ops.wm.append.

Tooltips

Use the tooltip that appears by default upon hovering of menu items / buttons: